Undecryptable symmetric encryption

Cryptography is necessary where the confidentiality of information needs to be protected, but it often not sufficient. There are many cases where a persistent attacker can obtain the secret keys in use on a system, no matter how much the system tries to prevent this (e.g. cold-boot attacks, DRM circumvention). On the other hand, there are many systems where information needs to be stored and protected, but not understood by (i.e. available to) the recording device. This includes sensors, video recorders, CCTV, networking equipment, etc. By the nature of symmetric encryption, they have more access than they need. Asymmetric encryption could be used to prevent this, but is impractical for large volumes of data. We present a solution that allows a device to symmetrically encrypt data without itself being able to decrypt it, and nor can any attacker that compromises the device.